That is potentially a dangerous and tragic situation that needs expert attention. That height is well beyond a homeowner designed solution. You need a soils engineer to do this, and you will need to file those plans with your muncipality as well.

And you need to move that playground equipment as that hill could go at any time. I live 30 miles southeast of Oso Washington where about a mile of hillside collapsed last spring taking out an entire neighborhood. Don't let your kids play down there if it's been raining as the soil is unstable, and could give way at any time.

Where is your property line?

Almost the top of the hill. There is about 6 inches at the top that is not mine.

I would suggest hiring a landscape architect to develop and derive a creative solution and involve a professional engineer (PE) who is experienced in both soils and structural wall design to provide the final design of the wall. Lastly, I would suggest having the landscape architect and professional engineer bid out the repair work for you to qualified contractors who are experienced constructing whatever type of retaining wall is designed for the yard.